This invention relates to a semiconductor device whose wiring layer is formed of a superconductive material.
Materials employed as electrodes or wirings of semiconductor devices, such as integrated circuits (hereinafter "ICs"), generally comprise a metal, such as aluminum, or a semiconductor material, such as polycrystalline silicon. In today's integrated circuits, it is essential to improve the operating speed and integration density of the ICs. To enhance integration density, the IC wiring must be as narrow as possible. However, even if an electrically good conductive metal is used for the wiring layer, wirings in the sub-micron meter range have increased electrical resistance. This causes time delays and degrades the signal propagation in the IC.
Recently, research and development of an electrically superconductive (hereinafter "superconductive") material has yielded promising ceramic materials. These materials have zero electrical resistance below a certain temperature, such as 77 K, the boiling temperature of the liquid nitrogen. Superconductive ceramic materials with good electrical characteristics even at this relatively high temperature have been reported as comprising yttrium (Y), barium (Ba), copper (Cu), and oxygen, as well as a compound comprising barium, lanthanum (La), copper, and oxygen. These compounds are first formed as a thin film layer which is then heat-treated in an oxygen-containing atmosphere so that the material of the layer becomes superconductive. Using this superconductive ceramic material as the wiring layer for an IC can make the electrical resistance of the wiring zero. Thus, signals or a power source voltage can reach its destination without delay or degradation of the signal. Use of a superconductive material is particularly desirable in a high electron mobility transistor (hereinafter "HEMT") because of their remarkably high speed operation at temperatures below 77 K.
However, a superconductive ceramic material cannot simply replace a metal layer Wiring formed of a normal conductive material; that is, a material that is not a superconductive material. Such non-superconductive materials are hereinafter collectively referred to as a normal metal, such as aluminum or gold. There are several problems preventing use of the superconductive ceramic materials, as discussed below.
(1) In the region where the superconductive ceramic material directly contacts a semiconductor material, atoms in the superconductive material reacts with atoms in the semiconductor material. Particularly, copper, which is abundant in the superconductive material, easily forms an eutectic alloy with silicon (Si) the semiconductor silicon substrate, and penetrates further into the silicon substrate.
(1-1) When copper atoms reach a p-n junction, they cause the breakdown voltage of the p-n junction to be lowered, the reverse current of the p-n junction to be increased, and the current gain of a bipolar transistor to be decreased. These undesirable side effects result from the shortened lifetime of carriers in the p-n junction, or because of carrier generation and recombination centers causing noise generation in the p-n junction.
(1-2) When the copper atoms reach a MOS (metal oxide semiconductor) structure, they lower the dielectric breakdown voltage of the structure.
(1-3) Ohmic contact is not always accomplished between the superconductive wiring layer and the semiconductor region. Thus, the junction has a rectifier characteristic or; that is, a high electrical resistance.
(2) The superconductive ceramic materials reported in public are mostly bulk type materials; that is, they have a random layer structure or electrically isotropic characteristics. However, when a superconductive material is formed in a thin film that is needed to fabricate a wiring layer for an LSI (large scale IC) by means of widely used epitaxial growth techniques (e.g. a molecular beam epitaxy (MBE) or a reactive sputtering method), the atoms or molecules are deposited in a particular orientation, i.e. the layered structure becomes anisotropic. With this particular orientation of the atoms/molecules, the superconductive material forms a layered structure extending parallel to the substrate's surface plane, for example, (1 1 0) on which the wiring layer is deposited also in the plane (1 1 0) (FIG. 8(a)). The layer is superconductive mainly in the direction of the substrate's surface plane direction (1 1 0), which is referred to hereinafter as horizontal direction. In other words, the electrical resistance in an orthogonal direction to the horizontal direction, i.e., across the thickness of the layers as indicated by an arrow B in FIG. 8(a), does not become zero as discussed above. In the strict sense of the word, the critical value of the superconductive current, in the thickness direction (arrow B) is much less than that in the horizontal direction.
Referring to practical examples, the above-described problems are hereinafter explained. FIG. 1 is a cross-sectional view of a sample general MOS transistor in which an electrode and wiring of the drain are formed of a superconductive ceramic material. The numeral 1 denotes a p-type silicon substrate; the numeral 2 denotes a silicon dioxide (SiO.sub.2) field isolation layer; the numeral 3 denotes a polycrystalline silicon gate electrode; the numeral 4 denotes an n+ type source region, the numeral 5 denotes an n+ type drain region; the numeral 6 denotes a SiO.sub.2 inter-layer insulation film; the numeral 7 denotes an aluminum source electrode; and the numeral 8 denotes a superconductive ceramic material electrode/wiring. In this sample, direct contact between the drain electrode/wiring layer 8 and the n+ type drain region 5 not only produces a high electrical contact resistance negating the beneficial advantage of using a superconductive material, but also produces an alloy of the materials that causes junction leakage.
FIG. 2 is a cross-sectional view of another sample MOS transistor with a typical aluminum drain electrode that connects the drain region to a superconductive ceramic material drain wiring. In FIG. 2, the same numerals denote the same parts or parts having the same function as those in FIG. 1. The numeral 8A denotes a drain electrode, and the numeral 8B denotes a drain wiring. In this sample, the reaction of the superconductive ceramic material with the n+ type drain region is prevented because the superconductive material 8B does not directly contact the n+ type silicon drain region 5. Furthermore, if the drain wiring 8B is anisotropic, the contact resistance increases because the aluminum electrode 8A contacts the superconductive wiring layer 8B at its horizontal surface where the superconductive property is not fully realized. Accordingly, the beneficial advantages of employing a superconductive ceramic material cannot be achieved.